Connected: An Internet Encyclopedia
RFC 1771 - 7. BGP Version Negotiation.

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### 7. BGP Version Negotiation.

BGP speakers may negotiate the version of the protocol by making multiple attempts to open a BGP connection, starting with the highest version number each supports. If an open attempt fails with an Error Code OPEN Message Error, and an Error Subcode Unsupported Version Number, then the BGP speaker has available the version number it tried, the version number its peer tried, the version number passed by its peer in the NOTIFICATION message, and the version numbers that it supports. If the two peers do support one or more common versions, then this will allow them to rapidly determine the highest common version. In order to support BGP version negotiation, future versions of BGP must retain the format of the OPEN and NOTIFICATION messages.

Next: 8. BGP Finite State machine.

Connected: An Internet Encyclopedia
RFC 1771 - 7. BGP Version Negotiation.